Many cancers arise from the gradual accumulation of genetic changes in cells. Gene therapy approaches as well as techniques for recognizing cancer cells with abnormal genes or elevated levels of certain mRNAs include the design and delivery into cancerous cells of antisense and antigene oligonucleotides or their synthetic mimics such as peptide nucleic acids (PNAs). PNAs are highly stable, resistant to nucleases and proteases, and bind RNA and DNA targets in a sequence-specific manner with high affinity. One of the main obstacles for gene therapy is a lack of technology for selective delivery of gene agents into cancer cells in vivo. Here we propose a new technology for selective delivery into cancer cells of PNAs targeting mRNAs involved in tumor growth and metastasis. It is well established that tumors develop a hypoxic and acidic extracellular environment, especially in the earlier stages. We designed a short peptide that is soluble in water and able to insert into the membrane as a transmembrane alpha-helix at low pH (<6.5) but not at normal pH (7.4). The peptide acts as a nanosyringe: it inserts in the membrane at low pH, translocates and releases in the cytoplasm various molecules, including dyes, toxins, and PNAs (Reshetnyak et al., PNAS, 2006, 103, 6460). The fluorescent PNAs are translocated into cells and stain the cytoplasm and nuclei. The mechanism of translocation of pH Low Insertion Peptides (pHLIPs) is based on a protonation of two Asp residues in the transmembrane domain, and this mechanism is fundamentally different from all reported peptide delivery agents. Whole-body imaging revealed that fluorescent pHLIPs accumulate in tumors in mice. The accumulation in tumors occurs because pHLIPs insert in the membrane at low pH while they interact only weakly with the surfaces of cells in tissues at normal pH. The replacement of two Asp residues by Lys or Asn residues eliminates the ability of pHLIPs to accumulate in tumors, which confirms the proposed mechanism of insertion of pHLIPs into cells. Our goal is to develop this nanosyringe technology for selective intracellular delivery of antisense and antigene PNAs into cancer cells in vitro and in vivo. We plan to conjugate various PNAs via disulfide linkages to the end of the peptide that inserts inside a cell. The efficiency of translocation of PNAs mediated by pHLIPs will be tested on different cell lines in vitro and in vivo in mice using fluorescence microscopy, flow cytometry, spectroscopy, whole-body imaging, and by measuring of level of expression of target proteins and rates of cell proliferation and tumor growth. The pHLIP nanosyringe could be a very effective tool for molecular analysis of cancer cells and diagnosis and treatment of cancer. [unreadable] [unreadable] [unreadable] [unreadable] [unreadable]